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ICJ 9020 



Bureau of Mines Information Circular/1985 



Design of Bulkheads for Controlling 
Water in Underground Mines 



By Gregory J. Chekan 




UNITED STATES DEPARTMENT OF THE INTERIOR 



'^/NES75TH AV^ 



Information Circular 9020 



Design of Bulkheads for Controlling 
Water in Underground Mines 



By Gregory J. Chekan 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




Library of Congress Cataloging in Publication Data: 



{ 




^ fll *° 



W 







^D 1 



Chekan, G« J. (Gregory J.) 

Design of bulkheads for controlling water in underground mines. 

(Information circular ; 9020) 

Bibliography: p. 23-24. 

Supt. of Docs, no.: I 28.27:9020. 

1. Mine drainage. 2. Mine water. 1. Title. II. Series: Informa- 
tion circular (United States. Bureau of Mines) ; 9020. 



-qP- N294uU4 [TN321] 622s [622'. 5] 84-600374 










a- 
X CONTENTS 

^ p ag e 

Abstract. 1 

Introduction 2 

Bulkhead design methods 3 

Types of bulkheads 3 

Factors to consider in bulkhead design 3 

Thin and thick plate designs 4 

Thin plate design 4 

Thick plate design 4 

Trench depth 5 

South African plug design 6 

Plug design formulas 6 

The relation of water leakage to plug length 10 

Single and double bulkhead seals 11 

Single bulkhead seals 11 

Double bulkhead seals 12 

Concrete specifications and placement methods 12 

Specifications 12 

Placement 14 

Pressure grouting 15 

Grouting materials 16 

Portland cement grout 16 

Chemical grouts 17 

Grouting methods 17 

Barrier pillars 18 

Pillar considerations 18 

Pillar width formulas 18 

Monitoring water pressure 20 

Obtaining MSHA approval 22 

Discussion 22 

References 23 

Bibliography 24 

Appendix A. — Hydrostatic testing of a single bulkhead seal 26 

Appendix B. — Flexural strength analysis for concrete block bulkhead 36 

ILLUSTRATIONS 

1. Methods of designing plugs to retain high water pressures 7 

2. Dimensions of experimental plug 9 

3. Plug constructed on the 1,200-ft level of the Friendensville Zinc Mine... 10 

4. Length of plugs based on ultimate pressure gradient values 11 

5. Double bulkhead seal 12 

6. Concrete being mixed underground and placed in forms by concrete pump.... 15 

7. Pumping concrete from the surface through a single vertical borehole to a 

/ central underground site 15 

Vjj 8. Pumping concrete from the surface through a vertical borehole directly to 

bulkhead site. 15 

\« A-l. Diagram of single bulkhead seal 26 

l A-2. Location of bulkhead in Safety Research Coal Mine 27 

V^ A-3. Bulkhead under construction 28 

A-4. Cross section of 1-in pipe grouted into concrete block 29 

,?) A-5. Diagram of test apparatus 29 



ILLUSTRATIONS — Continued 



Page 



A-6. 

A-7. 

A-8. 

A-9. 

A-10. 

B-l. 



1, 
2. 

3, 

A-l, 



Drill plan for injecting polyurethane grout in strata surrounding the 

bulkhead 30 

Packer-mixer assembly installed in borehole 31 

Polyurethane grout emerging from strata 31 

Diagram of standpipe, pressure gauge, and porous tube arrangement 34 

Polyethylene porous tube installed on the inby side of bulkhead 35 

Correction factor for bulkhead width-to-height ratios 36 

TABLES 

Summary of test results on experimental plug 8 

Parallel plugs of the Witwatersrand and Orange Free State gold fields 

for which there are records of loads applied of over 1,000 psi 9 

Ultimate compressive, tensile, and shear strengths for 1:2:4 concrete 

mix 13 

Test procedure for incrementally pressurizing bulkhead 29 





UNIT OF MEASURE 


ABBREVIATIONS 


USED IN THIS REPORT 




ft 


foot 


psi 


pound per square inch 




gal/h 


gallon per hour 


psig 


pound per square inch, 


gauge 


in 


inch 


V dc 


volt, direct current 




mA 


milliampere 


wt pet 


weight percent 




min 


minute 


yr 


year 




mm 


millimeter 









DESIGN OF BULKHEADS FOR CONTROLLING WATER IN UNDERGROUND MINES 

By Gregory J. Chekan 



ABSTRACT 

This Bureau of Mines report presents three methods for designing 
bulkheads to impound water underground: (1) thin and thick plate de- 
sign; (2) South African plug design; and (3) single and double bulkhead 
seal design. Related areas critical to the long-term effectiveness of 
underground water impoundments are also addressed. These include bulk- 
head anchorage, concrete specifications and placement, the grouting of 
permeable strata, and the sizing of barrier pillars. A case study 
involving hydrostatic tests conducted on a single bulkhead seal con- 
structed in the Safety Research Coal Mine of the Bureau's Pittsburgh 
Research Center is presented in an appendix. 



1 Mining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA, 



INTRODUCTION 



Bulkheads are commonly used to seal 
abandoned workings and protect adjacent 
active mines from explosion; however, 
bulkheads can also be used to control 
unwanted inflows of water. Ground water 
seepage from poorly sealed shafts, water- 
bearing strata, and abandoned mine areas 
used for impoundment are the major 
sources of water inflow. In some in- 
stances, water levels in these abandoned 
areas can rise rapidly or go completely 
undetected while the areas are accumulat- 
ing excessive hydrostatic pressure. This 
poses a potential inundation hazard to 
the active mine, especially if the bulk- 
head is not suitably designed to retain 
water. 

The Federal Coal Mine Safety and Health 
Act of 1977 requires that bulkheads which 
seal abandoned areas be "explosionproof " 
but makes no requirements on their abil- 
ity to perform as water seals. 2 The Bu- 
reau has conducted extensive research 
into the design and construction of ex- 
plosionproof bulkheads and the forces ex- 
erted upon them from coal dust and meth- 
ane ignitions. Although these designs 
may have application for impounding wa- 
ter, there are differences between ex- 
plosion pressures and hydrostatic pres- 
sures and the forces that they exert on a 
structure. 

In the case of an explosionproof bulk- 
head, the structure may never experience 
a significant loading until an explosion 
occurs. A methane or coal dust explosion 
exerts a dynamic loading on the bulkhead 
that rarely exceeds 50 psig. As a gen- 
eral rule, pressure at 200 ft or more 
from the origin of an explosion will not 
exceed 20 psig unless coal dust accumula- 
tions are abnormal and the incombustible 
content of the dust is far less than re- 
quired by law (1_). 3 In contrast, inunda- 
tion bulkheads are usually subject to a 
constant hydrostatic pressure, a static 

^Regulations governing the sealing of 
abandoned areas are covered in the Code 
of Federal Regulations, Title 30, Chapter 
1, Part 75, Subchapter D, Subparts 329-1, 
329-2, 330 and 330-1 . 



loading, which could be present for the 
entire life of the bulkhead. In extreme 
cases, this pressure may reach 500 psi 
(approximately 1,150 ft of waterhead) and 
last for several days, until pumping or 
draining operations can be initiated. 
Permeation of acid water is another major 
structural concern, for it deteriorates 
the bulkhead and its anchorage, as well 
as the ground around the bulkhead. 

When designing and constructing a bulk- 
head for the purpose of impounding water, 
several general criteria should be met: 

1. The bulkhead should be designed to 
withstand the static forces of hydro- 
static pressure rather than the dynamic 
forces of an explosion. 

2. The bulkhead should be constructed 
from a material, such as concrete, which 
will resist deterioration by water. 

3. The bulkhead should be constructed 
sufficiently thick and properly anchored, 
and the surrounding strata should be 
pressure grouted to minimize water 
seepage. 

The ability to safely impound water un- 
derground will become increasingly impor- 
tant in future years. Inundation bulk- 
heads will be needed to protect active 
workings in areas where mining is in 
close proximity to surface water bodies 
or water-bearing strata. Mining compa- 
nies are also beginning to examine the 
possibility of impounding water under- 
ground as a means of eliminating the 
costly treatment of acid mine water be- 
fore discharge. Presently, there is no 
commonly accepted design method for con- 
structing bulkheads for this purpose. 
Prior to constructing a bulkhead for im- 
pounding water, a mine operator must 
first notify the Mine Safety and Health 
Administration (MSHA) and then submit de- 
tailed design and construction plans for 

•^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendixes. 



approval. This usually requires the as- 
sistance of a professional engineering 
consultant who has had prior experience 
in this subject. 

It is strongly emphasized that this re- 
port is not intended to serve as a com- 
plete guide to the design of bulkheads 
for impounding water underground. Pub- 
lished literature on this subject is 



marginal, and more research needs to be 
conducted to add to our present knowledge 
of plug and bulkhead design. This report 
seeks to increase the mine operators' un- 
derstanding of inundation bulkhead design 
and other problems associated with under- 
ground water impoundments. Such informa- 
tion is essential to safe mining in areas 
where inundation is possible. 



BULKHEAD DESIGN METHODS 



TYPES OF BULKHEADS (2) 

Bulkheads constructed for impounding 
water can be classified into five types. 

Control . — Bulkheads that are planned in 
advance and constructed to seal abandoned 
mines and prevent the inflow of water in- 
to an adjacent active mine. They are in- 
stalled between barrier or chain pillars 
with no means of access to the sealed-of f 
area. Pipes with valves are usually cast 
into the bulkhead to measure and control 
water levels. They are designed to with- 
stand the maximum water pressure that can 
develop. This is usually equal to the 
depth of the bulkhead below the surface. 

Emergency . — Bulkheads constructed under 
emergency conditions to seal off unex- 
pected inrushes of water. They are de- 
signed to withstand the maximum water 
pressure that can develop, with no means 
of access to the sealed-off area. 

Precautionary . — Bulkheads planned in 
advance and constructed in main entries 
and haulage roads to control flooding 
should an inundation occur. Watertight 
doors are cast into the bulkheads to pro- 
vide a travelway for workers and equip- 
ment. These bulkheads are designed to 
withstand the maximum water pressure that 
can develop. 

Consolidation . — Bulkheads constructed 
to control water inflow during high-pres- 
sure grouting and ground consolidation 
operations. They are temporary struc- 
tures which are removed after ground 
sealing is completed. 

Open Dam Walls . — Open dam walls are 
used to impound water for treatment or 
conservation and reuse. They do not seal 



the entire entry and simply limit the 

height of water to the height of the dam; 

but large volumes of water can be im- 
pounded in this way. 

FACTORS TO CONSIDER IN BULKHEAD DESIGN 

Several factors should be considered 
before designing and constructing a bulk- 
head to impound water: 

1. The bulkhead should be located in 
competent ground that is not excessively 
fractured or broken, preferably in areas 
of stable ground ( 1_) . However, in most 
coal mines ground movements such as roof 
convergence and floor heave are inevit- 
able, and supplemental roof supports 
(timbers and cribs) should be installed 
at the site. 

2. The bulkhead, in most cases, should 
be designed to withstand the maximum hy- 
drostatic pressure that can develop. 
Practical limits of potential inundation 
can be determined by plotting on a coal 
contour map the expected mine pool ele- 
vations and corresponding ground surface 
elevations. Areas where excessive water- 
heads may accumulate can then be pro- 
jected. To convert waterhead, which is 
expressed in feet, to hydrostatic pres- 
sure, which is expressed in pounds per 
square inch, multiply the waterhead by 
0.434. 

3. The concrete for constructing the 
bulkhead must be properly mixed and 
placed to achieve acceptable strengths 
upon curing. (See section on "Concrete 
Specifications and Placement Methods.") 



4. Anchorage of the bulkhead to mine 
roof, ribs, and floor is important, and 
depends on design as well as on strata 
type and condition. Some design methods 
rely on the strength of the concrete 
bearing against the irregularities in the 
rock surface to provide anchorage. Oth- 
ers require the excavation of trenches. 
Anchorage requirements for each design 
method are discussed in their respective 
sections. 

5. Adequate pressure grouting of the 
immediate strata surrounding the bulkhead 
is probably the most significant factor 
in the bulkhead's long-term performance. 
Deterioration of the anchoring strata by 
acid water permeation is a major struc- 
tural concern, especially if large pres- 
sures are anticipated over the life of 
the bulkhead. A brief review of grouting 
materials and methods frequently used to 
seal coal mine strata is given in the 
section on "Pressure Grouting." 

THIN AND THICK PLATE DESIGNS (3) 

Thin and thick plate formulas for de- 
signing bulkheads are derived through 
static analysis techniques, but the ef- 
fectiveness of these designs for impound- 
ing water has not been thoroughly evalu- 
ated through full-scale prototype tests. 
These designs apply only to bulkheads 
constructed from homogeneous and isotrop- 
ic materials. 

Thin Plate Design 

This design assumes that the bulkhead 
is to act as a simply supported thin 
plate, spanning the width of the entry; 
its structural behavior under static load 
is characterized by bending at midspan. 
Under these conditions, bending failure 
is governed by the tensile strength of 
the construction material. Using this 
analysis, the required bulkhead thickness 
is predicted to be 



T = 0.865 a /p/f + 



where T = bulkhead thickness, ft; 



(1) 



a = maximum entry dimension, ft; 

p = hydrostatic pressure, psi; 

and f + = allowable tensile strength of 
construction material, psi. 

If a = 18 ft, p = 100 psi (230 ft of wa- 
terhead) , and f + (for concrete) = 150 psi 
(1_) , a bulkhead approximately 12.7 ft 
thick and unanchored 4 would be needed to 
impound 230 ft of water. 

In practice, this design formula may be 
conservative. Research indicates that 
bulkheads designed accordingly have re- 
sisted much higher dynamic loads (explo- 
sion pressure) with a considerable margin 
of safety (1_) . This observation suggests 
not only that bulkheads designed by this 
method can resist a much higher dynamic 
load, but also that the design method 
could be modified to more realistically 
represent the response of the structure 
under static load. Such is the case in 
thick plate design. 

Thick Plate Design 

The Bureau conducted a series of model 
test in the early 1930 's (_4-5_) and found 
that restraining the edges of a bulkhead 
caused a dramatic increase in strength, 
well beyond what was expected from plate 
theory. Full-scale explosion tests also 
showed that bulkheads that were recessed 
into the roof, ribs, and floor, and that 
had thickness-to-width ratios of at least 
1 to 10, resisted much higher pressures 
than the design pressure. It was con- 
cluded that recessing the ends of the 
bulkhead into the surrounding strata al- 
lows the structure to act as a flat arch. 
Under load, this arching behavior trans- 
mits a lateral thrust to the strata, 
which then act as a buttress. 

Attempts have been made to explain this 
arching behavior through static design 
models. Whitney (6) developed an arch 
model that assumed the bulkhead to fail 
as two rigid walls , fractured at both 

^Anchorage is supplied by bearing re- 
sistance between rock and concrete. 



sides and along the midspan. Using this 
assumption, the design formula is pre- 
dicted to be 



where 



T = pa /p/f c (2) 

T = bulkhead thickness, ft; 

a = maximum entry dimension, ft; 

p = hydrostatic pressure, psi; 

f c = allowable compressive 

strength of construction 
material, psi; 



-Tf 1 " 



S^ A + 4y 2 
E /l + 4y 2 - 1 



where E = Modulus of elasticity of 
construction material, psi; 

S c = ultimate compressive 

strength of construction 
material, psi; 

and y = T/a, thickness-to-width 
ratio. 

For average concrete mixtures; S c s 3,000 
psi, E = 3 million psi and with thick- 
ness-to-width ratios (y) of at least 1 to 
10, p is ~0.670. Therefore, the thick- 
ness formula becomes 



T = 0.670 a /pTf^ 



(3) 



If a = 18 ft, p = 100 psi (230 ft of 
waterhead) , and f c (for concrete) = 1,000 
psi, a bulkhead approximately 3.8 ft 
thick and firmly anchored (recessed into 
the roof, ribs, and floor) would be 
needed to impound 230 ft of water. 

Design equation 3 is very similar to 
design equation 1, the major difference 
being that the allowable tensile strength 
(ft) is replaced by the allowable com- 
pressive strength (f c ). For most materi- 
als, f c is 5 to 10 times f t . This allows 
a reduction in required design thickness 
of 50 to 70 pet, provided that there is 
adequate anchorage. Excavating trenches 



to recess the bulkhead into the roof , 
ribs, and floor contributes to this in- 
creased strength. The trenches assure 
that the applied load develops through 
the bulkhead and is then transferred to 
the load-bearing capacity of the coal 
roof, ribs, and floor. 

The thick plate design approach has two 
principal drawbacks. First, the arching 
behavior described earlier does not occur 
until there is considerable cracking or 
fracturing of the bulkhead. The failure 
of a bulkhead under these circumstances 
can be catastrophic, especially if the 
hydrostatic pressure exceeds the design 
pressure. Second, the strength of the 
bulkhead depends directly on the bearing 

strength of the coal, roof, ribs, and 
floor strata. The Bureau has conducted 
research along these lines to determine 
the compressibility and bearing strength 
of in-place coal (_5, _7 ) . Future design 
criteria should include the bearing 
strengths of the coal, the roof, and the 
floor to assure adequate design. 

Trench Depth (1_, 4.) 

The required trench depth to properly 
anchor bulkheads has not yet been deter- 
mined through either model or full-scale 
tests. However, acceptable requirements 
for minimum trench depths for bulkheads 
less than 3 ft thick can be presumed 
from research conducted on explosionproof 
bulkheads . 

For concrete bulkheads, Rice (4-5) > 
recommended trench depths, in the coal 
ribs, of at least one-tenth the width of 
the entry (0.1 W) after all loose coal on 
either rib had been scaled away. Howev- 
er, if the coal is distinctively soft or 
broken, a trench depth of one-fifth the 
width of the entry (0.2 W) was advised. 
In accordance, Mitchell (1_) recommended 
that rib trench depths be at least 2 ft 
or the thickness of the bulkhead, which- 
ever is greater. 

Floor trenches should be a minimum of 
12 in deep, provided the immediate floor 
strata have not been softened by water. 
If this is the case , trenching should 



proceed until a competent stratum is 
reached. After excavation, holes should 
be drilled along the centerline of the 
trench to accommodate steel reinforcing 
rods of at least 7/8-in diam and 38 in 
long. The steel rods should be firmly 
grouted no less than 18 in deep, and 
spaced at no more than 18-in intervals. 

If feasible, trenching of the roof is 
recommended. Owing to roof sag, the roof 
is usually where most water seepage will 
occur. Trenching the roof may be a dif- 
ficult task because of the unpredictable 
nature of most roof rock. The immediate 
area should be stabilized with supplemen- 
tal supports, and care should be exer- 
cised during the trenching operation. 
Trenches should be cut at least 8 in 
deep. Once the trenches are complete, 
additional anchorage should be provided 
with steel reinforcing rods of at least 
7/8-in diam and 30 in long. The steel 
rods should be firmly grouted into the 
roof along the centerline of the bulkhead 
at a depth of no less than 18 in, with no 
more than 18-in spacing between rods. 

When excavating the trenches the fol- 
lowing should be observed: 

1. Select a site where the ribs, roof, 
and floor are competent and not affected 
by long-term weathering or excessive 
ground movement and stress. As a routine 
measure, supplemental supports should be 
installed at the selected site. 

2. Trim all loose coal from the ribs, 
making them as straight as possible. The 
same applies for loose rock on the floor 
and roof. 

3. Cut the trenches with hydraulic or 
pneumatic tools, taking care to avoid un- 
necessarily fracturing the strata. Ex- 
plosives should not be used to excavate 
the trenches unless very hard, competent 
strata are encountered. 

4. Keep the width of the trenches 
the same as that of the bulkhead. All 
trenches should be cut as square as pos- 
sible, especially at the inner and outer 
corners where the floor and rib trench 
meet. 



SOUTH AFRICAN PLUG DESIGN (2, 8-_9) 

South African plug research was con- 
ducted during the late 1950' s and early 
1960's to resolve inundation problems 
encountered in the mines of the Witwa- 
tersrand and Orange Free State gold 
fields. W. S. Garrett and L. T. Campbell 
Pitt (8-9) designed, constructed, and 
tested an experimental concrete plug that 
withstood a hydrostatic pressure in ex- 
cess of 6,000 psi. Their research led to 
a better understanding of the criteria 
that influence plug design. Although 
their tests were conducted under condi- 
tions unique to deep gold mines of South 
Africa, the assumptions and theory which 
formed the basis for their design formula 
can also be applied to water impoundments 
in underground coal mines. 

Plug Design Formulas 

Garrett and Campbell Pitt considered 
three possible methods for designing 
plugs to retain high water pressures 
(fig. 1): 

1. The plug would be constructed as a 
slab with all four sides recessed into 
the rock, with or without steel rod rein- 
forcement (plate design). 

2. The walls of the drive would be 
tapered so the load could be transmitted 
from the plug to the rock wall by 
compression. 

3. The plug would be parallel to the 
walls of the drive, and there would be no 
need for recessing or tapering. Anchor- 
age would be provided by the concrete 
bearing, against the irregularities in 
the rock surface, after all loose materi- 
al had been scaled away. 

In many instances, these plugs had to 
be constructed under emergency condi- 
tions. Time was the most important fac- 
tor, and site preparation had to be mini- 
mal. For this reason, they choose the 
third method (parallel plug design), 
rather than the first two methods, both 



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Taper plug 



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Parallel plug 

KEY 

^ Direction of 

hydrostatic pressure 

Not to scale 

FIGURE 1. - Methods of designing plugs to re- 
tain high water pressures. Adapted from W. S. 
Garrett and L. T. Campbell Pitt (8). 

of which required ground excavation be- 
fore the plug could be installed. 

At that time, the accepted theory for 
parallel plug design assumed that the 
load, induced by hydrostatic pressure, 
was transmitted from the concrete plug to 
the rock as punching shear around the 
perimeter of the plug and along its 
full length. This was accepted by the 



Government Mining Engineer as adequate 
criteria for parallel plug design. The 
length of the plug was derived from 



Therefore: 



pab = 2(a+b)lf s 

I = P ab 



(5) 



2(a+b)f s 

where 1 = length of the plug, ft, 

a = width of the entry, ft, 

b = height of the entry, ft, 

p = hydrostatic pressure, psi, 

and f s = allowable shear stress for 
rock or concrete, whichever 
is the lesser, 5 psi. 

To learn more about this design, Gar- 
rett and Campbell Pitt constructed a 
smaller experimental plug, with doors, 
and tested it hydrostatically ( 8_) . The 
dimensions of this experimental plug are 
shown in figure 2. With each test, the 
hydrostatic pressure was increased and 
the strata surrounding the plug were 
pressure-grouted more extensively to seal 
leaks. A summary of the test results is 
shown in table 1. Upon completion of the 
test Garrett and Campbell Pitt made sev- 
eral observations, on which basis they 
revised the parallel plug design under 
the assumption that when a plug is in- 
stalled in an entry, half of the rock 
surfaces will be at an angle (45°) that 
would resist movement of the plug by com- 
pression. The other half of the rock 
surfaces would resist movement by ten- 
sion, provided the contact between rock 
and plug was ensured through adequate 
pressure grouting. The length of the 
parallel plug was represented as 



-^The Government Mining Engineer of 
South Africa recommends 85 psi as the al- 
lowable shear stress (f s ) for concrete 
placed in the normal manner and 1 20 psi 
for plugs where positive contact between 
rock and plug is ensured by subsequent 
pressure grouting (2). 



Therefore 
where 



pab = 2(a+b)l/2 1 tan 45°f c (5) 
pab 



1 = 



(a+b)f c 

1 = length of the plug, ft, 

a = width of the entry, ft, 

b = height of the entry, ft, 

p = hydrostatic pressure, psi, 

and f c = allowable compressive 

strength of the rock or 
concrete, whichever is the 
lesser, 6 psi. 

Garrett and Campbell Pitt realized that 
the two formulas were oversimplifications 

°Garrett and Campbell Pitt used 600 psi 
as the allowable compressive strength of 
concrete. 



of a very complicated stress condition. 
They felt that movement of the rock, from 
stress induced by the hydrostatic load, 
might likely be the governing factor in 
assessing the stress condition in the 
concrete of the plug itself. Therefore, 
the values that could be set on either 
allowable shear stress or allowable com- 
pressive strength depend on the effec- 
tiveness with which the concrete of the 
plug is confined by the surrounding rock. 
Using this analysis, it is impossible to 
recommend a particular formula for as- 
sessing the stress condition in the con- 
crete until some satisfactory quantita- 
tive results can be produced on what 
actually takes place in the rock sur- 
rounding a plug under a hydrostatic load 
(9). 

Table 2 gives a list of parallel plugs, 
constructed in the Witwatersrand and 
Orange Free State gold fields. Garrett 
and Campbell Pitt calculated the respec- 
tive values of f c and f,~ from the two 



TABLE 1. - Summary of test results on experimental plug 



Test 


Date 


Cementation 


Pressure, 
psi 


Remarks 


1 


7/31/57 


No cementation. 


75; 
200; 
310. 


Heavy leakage on rock and concrete 
contacts particularly at hanging; 
tappings on hanging contact closed 
off to build up pressure >75 psi. 


2. . .. 


8/15/57 


Rock and concrete 


650- 


Leakage on the rock and concrete 




to 


contacts cementated 


1,750 


contacts reduced; leakage past 




9/ 5/57 


at 3,000 psi. 




plug 50 gal/h at 1,750 psi. 


3 


9/12/57 


Rock surrounding the 


1,800; 


Total leakage past plug 156 gal/h 






plug cementated at 


2,500. 


at 1,800 psi; 300 gal/h at 2,500 






6,000 psi. 




psi. 


H • • • • 


10/ 3/57 


Leaks sealed by 
cementation. 


4,300 


An old diamond drill hole began to 
leak at 3,000 psi; leakage 128 
gal/h at 4,300 psi; leakage 
stopped when pressure was reduced 
to 2,000 psi. 


5 


10/ 8/57 


Further cementation 
to seal leaks. 


5,700 


Leakage not measured; pipe sleeve 
corrugated with crests of corru- 
gations 15 in, 27 in, and 37 in 
from the door face. 




10/15/57 




6,200 


Leakage in footwall of the drive 














~400 gal/h; no further distortion 










of the pipe sleeve was apparent. 




10/17/57 


Footwall leak 
cementated. 


6,800 


Leakages in footwall and hanging 
of main drive; pressure could not 
be raised further. 



Source: W. S. Garrett and L. T. Campbell Pitt (8), 




25' 



jjaigjassflsgasaiaa^ga; 



.. ijl J— r — — -, , .,. U 



xV 



7' 8^2" 




Water 
chamber 



l2"l2"l2"l2' J l2"l2"l2^ 8 l/ 2 " Ca ^J eel 



PLAN, sectional 



r,Wi\V" i Wff M ""\\ritltiWf, l 



Drive 



"iWAwy^Hwrnv^iW 1 * 1 




_*. Mild steel tube 
A I with welded flanges 



4' approx 






tti, 26 diam 
||i,Un,i,| 



||'"i Mill | T || 

AU 




ELEVATION, general arrangement SECTION A-A 

FIGURE 2. - Dimensions of experimental, plug. Adapted from W. S. Garrett and L. T. Campbell Pitt 
(8). 

TABLE 2. - Parallel plugs of the Witwatersrand and Orange Free State gold 
fields for which there are records of loads applied of over 1,000 psi 





Dimens 


ions 


,' ft 


Pressure, 
psi 


fs> 
psi 


fc 
psi 


Pressure 


Mine and location 


H 


W 


L 


gradient , 
psi/ft 


Free State Geduld: No. 2 Shaft.. 
West Driefontein: 


47 

4 
13 
13.5 

12.25 
12.25 


11 

4 
10 
10 

11 
11 


100 

7.6 
41.6 
63 

36 
12 


2,250 

6,800 
1,827 
1,650 

1,340 
1,340 


100 

885 

124 

75 

108 
324 


200 

1,790 
248 
150 

216 

648 


22.5 
887.0 




43.5 


Virginia-Mer rie : 


26.2 
39.2 




111.7 



'H = height; W = width; L = length. 
Source: W. S. Garrett and L. T. Campbell Pitt (9), 



design formulas. Note the high values of 
f s and f c for the West Driefontein exper- 
imental plug, the plug that formed the 
basis for their design assumptions. 

A plug similar to those constructed in 
South Africa is shown in figure 3. This 
particular plug is situated on the 1,200- 
ft level of the Friendensville Zinc Mine 
(owned by Gulf and Western Industries) , 



located near Allentown, PA. This precau- 
tionary plug separates the main shaft 
from the stopes. In the event of an in- 
undation, the watertight doors are closed 
to prevent the main shaft from flooding. 7 



7 For more information on the Friendens- 
ville Mine, see Cox (10). 



10 




FIGURE 3. - Plug constructed on the 1,200-ft level of the Friendensvi lie Zinc Mine. 



The Relation of Water Leakag e 
to Plug Length 

The effectiveness of a plug to impound 
water depends on the ability to minimize 
water leakage. Water can leak past a 
plug in several ways: along the plug- 
rock interface; through the cracks, frac- 
tures, and fissures in the rock surround- 
ing the plug; or through the concrete of 
the plug itself. These three modes of 
water leakage are dependent upon the 
length of the plug and the resistance of 
the rock to the permeation of water. 
Garrett and Campbell Pitt felt that the 
leakage aspects of the rock could be used 
as criteria for plug design. They ex- 
pressed this leakage as a pressure gradi- 
ent in the rock: 

p.g. = p/1 (6) 

where p.g. = pressure gradient, psi/ft, 



p = hydrostatic pressure, psi, 

1 = length of plug, ft. 

According to the tests on the experi- 
mental plug in table 1, the limiting val- 
ue of the pressure gradient was achieved 
on four occasions when leakage became ex- 
tensive and obvious: 

1. Before grouting of the plug-rock 
interface when hydrostatic pressure 
reached 75 psi, the pressure gradient was 
75 psi/7.67 ft = 9.8 psi/ft. 

2. After grouting the plug-rock inter- 
face but before grouting the rock, vrhen 
the hydrostatic pressure reached 1,750 
psi the pressure gradient was 1,750 psi/ 
7.67 = 228 psi/ft. 

3. After grouting the rock to 6,000 
psi with cement, an approximate 



11 



hydrostatic pressure of 3,000 psi was ex- 
erted. The pressure gradient was ±3,000 
psi/7.67 ft = ±400 psi/ft. 

4. After extensive chemical grout in- 
jections in the rock, the hydrostatic 
pressure rose to 6,800 psi and the pres- 
sure gradient was 6,800 psi/7.67 ft = 887 
psi/ft. 

The pressure gradients calculated 
above are unique to the particular rock 
(quartzite) in which the experimental 
plug was constructed, but the theory of 
using a safe pressure gradient as design 
criterion offers a valuable means of tak- 
ing into account the important factor of 
the ground surrounding the plug. Garrett 
and Campbell Pitt believed that values 
for minimum and maximum pressure gradi- 
ents could be established experimentally 
for various rocks. Also, they recom- 
mended that plugs designed accordingly 
should have a leakage factor of safety 
of at least 4 and as much as 10 in some 
cases, depending on conditions such as 
fractures in the rock after mining and 
the subsequent redistribution of stress, 
porosity of the rock, and its acceptance 
of grout. 

Figure 4 shows the minimum length of 
plugs based on ultimate pressure gradient 
values obtained from the tests on the ex- 
perimental plug. Also given are curves 
for plugs with safety factors of 4, 6, 8, 
and 10 with the provision that the sur- 
rounding rock is grouted to at least the 
same pressure which the plug is designed 
to resist. 

SINGLE AND DOUBLE BULKHEAD SEALS (11-13) 

Single and double bulkheads seals , also 
known as hydraulic seals , are commonly 
used to permanently seal abandoned drift 
and slope mines and protect the environ- 
ment from the undesirable effects of acid 
mine drainage. Historically, thickness 
and anchorage requirements for these 
seals have been derived from experience 
and are based on the immediate ground 
conditions and the amount of water to be 
impounded. Various types of hydraulic 
seals have been demonstrated in the 
United States. Most of this sealing work 




2 4 6 8 10 12 14 16 
WATERHEAD (H), I0 3 ft 

FIGURE 4. - Length of plugs based on ultimate pres- 
sure gradient values. A, Minimum length of plug that 
would be required if the contact between plug and rock 
is ungrouted. No factor of safety; B, minimum length 
when the contact is grouted but before the rock is grouted. 
No factor of safety; C, minimum length when normal grout- 
ing of rock was at 6,000 psi. No factor of safety. {AC, 
e.g., means 4 x C); D, similarto C, but with the addition 
of chemicals to seal rock fissures. C is then applicable 
to a normally grouted plug but with no factor of safety. 
Adapted from W. S. Garrett and L. T. Campbell Pitt (8). 



was performed in the East, as part of 
Federal and State acid mine drainage re- 
search and abatement programs ( 11 , 13) . 

Single Bulkhead Seals 

Single bulkhead seals are usually con- 
structed from concrete, grouted aggre- 
gate, or concrete block. They are com- 
monly used to seal off abandoned mines 
from active workings, and such seals have 
been documented to withstand water pres- 
sure as high as 70 psi (161.5 ft of 
waterhead) (11). In many instances, 
plate theory is used in their design, 8 
or minimum thicknesses and anchorage 

8 Thick plate design, discussed earlier 
in this report, is applicable for deter- 
mining thickness and anchorage require- 
ments for single bulkhead seals. 



12 



requirements are derived from research on 
explosionproof bulkheads. The Bureau 
recommends that a bulkhead must withstand 
a dynamic pressure of at least 50 psi for 
it to be explosionproof 0_). 

To learn more about the application of 
explosionproof designs for impounding 
water, the Bureau hydrostatically tested 
a concrete block bulkhead commonly used 
in coal mines to resist explosion. The 
bulkhead is 16 in thick with the block 
laid in a transverse pattern and a pi- 
laster at center span for additional sup- 
port. It withstood 50 psi of water pres- 
sure before tests were stopped owing to 
water leakage through the bulkhead struc- 
ture. Details of this research are pre- 
sented in appendix A. 

There are numerous problems associated 
with the impoundment of water by single 
bulkhead seals. The long-term effective- 
ness of these seals is questionable even 
under low pressure, because of their rel- 
atively short thicknesses. When large 
hydrostatic pressures are anticipated, 
double bulkhead seals are considered more 
effective. 

Double Bulkhead Seals 

The double bulkhead seal is constructed 
by placing two retaining bulkheads in a 
mine entry and then placing an imperme- 
able seal in the space between them (fig. 
5). The front and rear bulkheads pro- 
vide a form for the center seal which is 
formed by placing concrete or injecting 
grout into a preplaced aggregate. The 
retaining bulkheads are constructed from 
concrete, concrete block, or brick and 
should be sufficiently thick and well an- 
chored to hold the center seal in place 
while it is poured and as it cures. 

The required thickness for double bulk- 
head seals range from 10 to 45 ft de- 
pending on the ground conditions, the 
strength of the concrete, and the amount 
of water to be impounded. There are no 
recommended designs for calculating this 



Not to scale 




FIGURE 5. • Double bulkhead seal showing re- 
taining bulkheads and concrete center. 

thickness , but the South African plug 
design may be applicable in this situa- 
tion. Since no trenching is required for 
the center seal, the anchorage depends 
on the strength of the concrete bearing 
against the irregularities in the rock 
surface. However, steel rods can be 
grouted into the roof, ribs, and floor 
surrounding the center seal to provide 
additional shear strength. 

The double bulkhead method of sealing 
mine entries has been successfully demon- 
strated at Moraine State Park, Butler 
County, PA, under the State's "Operation 
Scarlift" reclamation program. These 
seals were placed in inaccessible mine 
entries through vertical boreholes 
drilled from the surface. A total of 69 
seals were installed ranging in thickness 
from 17 to 40 ft. The seals were con- 
structed from a fly ash, sand, gravel, 
and cement mixture (12). 



CONCRETE SPECIFICATIONS AND PLACEMENT METHODS 



SPECIFICATIONS 

Concrete for constructing bulkheads can 
achieve different strengths depending 



upon the method of construction, the 
thickness of the bulkhead, the contents 
and proportions of the mix and curing 

time. 



13 



For explosionproof bulkheads the Bureau 
recommends (1_, 4-5) a mix, by volume, of 
1 part type I Portland^ cement, 2 parts 
clean sand, and 4 parts clean gravel. 
Only enough water should be added to make 
the mix homogeneous and give a stiffness 
consistency that will enable it to be 
properly placed in the form. Overwa- 
tering must be avoided, for it reduces 
the strength of the concrete. The Bu- 
reau contracted a professional engineer- 
ing laboratory to conduct strength tests 
on the above mix. Test cylinders were 
prepared, and after curing for 28 days 
the samples were tested for ultimate com- 
pressive and tensile strengths. Shear 
strength was calculated by using an equa- 
tion developed from Mohr's Circle which 
relates compressive, tensile, and shear 
strength of the sample. The strength 
values are given in table 3. 

TABLE 3. - Ultimate compressive, 
tensile, and shear strengths 
for 1:2:4 concrete mix 





Test 1 


Test 2 


Average 


Strength, psi: 








Compressive. . . 


2,933 


3,074 


3,004 




233 


255 


244 




NAp 
NAp 


NAp 
NAp 


766 




1.5 



NAp Not applicable. 

Equations 1-5 require using the allow- 
able flexural, shear, and compressive 
strength values of the construction 
material so as to provide adequate mar- 
gins of safety. As a rule of thumb, al- 
lowable strength values range from 20 to 
30 pet of the ultimate strength. Accord- 
ing to Garrett and Campbell Pitt (_9) , 
concrete of great strength is not impor- 
tant. If 2,500 psi ultimate compressive 
strength is obtained after 28 days, the 
safety factor is >4. They used 600 psi 
as the allowable compressive strength 
(f c ) for concrete plugs designed accord- 
ing to equation 5. In addition, plug 
design equations 4 and 5 require that the 

^Reference to specific equipment (or 
trade names or manufacturers) does not 
imply endorsement by the Bureau of Mines . 



allowable shear or compressive strengths 
of the concrete or rock be used. In most 
cases, the allowable shear and compres- 
sive strengths of coal and other strati- 
fied rock will be less than those for 
concrete and should be used in these de- 
sign equations to assure adequate margins 
of safety. 

During the curing of a large mass of 
confined concrete, such as a plug, crack- 
ing and shrinkage can occur. For this 
reason, prolonged, thorough curing is a 
significant factor in attaining imperme- 
able watertight concrete. Cracking is 
usually caused by high heat of hydration 
generated during curing. This weakens 
the concrete and may affect its ability 
to resist design pressure. Shrinkage can 
affect anchorage and is a result of ex- 
cessive water content or inadequate ag- 
gregate composition. Some shrinkage is 
inevitable in concrete, and pressure 
grouting is necessary to improve contact 
between the bulkhead and surrounding 
rock. The addition of pozzolans, such 
as fly ash, to concrete can improve work- 
ability, reduce heat of hydration and 
shrinkage, and increase resistance to 
sulfates contained in water. However, 
caution must be exercised in the selec- 
tion of pozzolans, because their proper- 
ties vary widely and excessive amounts 
may have adverse effects on the concrete, 
such as increased shrinkage and reduced 
strength and durability (14) . Before se- 
lecting a mix, trial mixes should be 
made, especially when using admixtures 
and pozzolans. 

To attain concrete with specific prop- 
erties, other types of Portland cement 
can be used. Type II is for general use, 
more specifically when moderate sulfate 
resistance and heat of hydration are de- 
sired. Type IV gives low heat of hydra- 
tion, and Type V is used when high sul- 
fate resistance is desired. Standard 
specifications for Portland cement are 
given in ASTM Designation CI 50, Part 14. 
Standard specifications for fly ash and 
raw or calcined natural pozzolans for use 
as mineral admixtures to cement concrete 
are given in ASTM Designation C618, Part 
14. 



14 



PLACEMENT 

There are several commonly used methods 
for placing concrete for bulkheads in ac- 
cessible mine entries. When constructing 
only one or two bulkheads , concrete can 
be mixed underground by hand or machine 
and placed in the forms either manual- 
ly or with concrete pumps (fig. 6). If 
several larger bulkheads are to be con- 
structed, concrete is usually placed 
from the surface through a vertical bore- 
hole to a central underground site where 
a slurry-distributor is located. The 
slurry-distributor remixes the wet con- 
crete and pumps it to the individual 
bulkhead sites (fig. 7). Another varia- 
tion of this method requires the drilling 
of one or several vertical boreholes di- 
rectly to the entry where the bulkhead(s) 
are to be placed. This method is used 
when several large plugs or double bulk- 
head seals are needed. Retaining bulk- 
heads or rigid wood forms are then built 
between the boreholes , and concrete is 
then introduced directly from the surface 
through the boreholes to the space be- 
tween the forms (fig. 8). 

When placing large volumes of concrete, 
such as a plug, the coarse aggregate may 
separate from the mix and settle, causing 
the concrete to lose its strength charac- 
teristics upon curing. To avoid this and 
also to improve concrete workability, the 
coarse aggregate can be preplaced between 
the retaining bulkheads or forms and con- 
crete grout injected into the aggregate. 
The aggregate can be preplaced by hand or 
stowed pneumatically . 1 

For bulkheads less than 3 ft thick ( J_) , 
concrete can be mixed underground by hand 
or machine and forms constructed from 
lumber. Plywood forms should be at least 
3/4 in thick and support boards 2 in by 4 
in. Boards must be properly spaced so 
the forms can resist the hydraulic head 
of the concrete as it is placed and 
cures. The ends of the forms should be 
flush with the edges of the trenches in 
the roof, ribs, and floor and should not 
extend into them. On the outby side (the 

10 Details for building seals and plac- 
ing aggregate by pneumatic stowing are 
provided by Maksimovic (15-16). 



side from which the concrete is placed) , 
the form boards should be about half 
height with the remainder of the boards 
cut and readily available for insertion 
as needed. 

Before placing the concrete, any water 
in the floor trench must be removed. The 
concrete is placed in successive horizon- 
tal layers , with care taken to fill the 
rib recesses completely. There must be 
no delay greater than 30 min between mix- 
ing and placing the concrete for the en- 
tire bulkhead, because greater delays 
cause cold joints that could weaken the 
structure. However, if unavoidable de- 
lays are foreseen, steel reinforcing 
rods, of at least 7/8-in diam and 16 in 
long, should be installed vertically in 
the last layer about 18 in apart and pro- 
jecting upward 8 in or more. The surface 
of the cold joint should be cleaned be- 
fore placing fresh concrete. As the roof 
is approached, placing the concrete be- 
comes more difficult but cannot be ne- 
glected. Concrete should be worked well 
into the roof cavity and also around the 
reinforcing rods extending from the roof 
and into recesses in both coal ribs. 

Forms should not be removed for at 
least 4 to 7 days after the concrete is 
placed. Noticeable voids on the out- 
by side of the bulkhead can be filled 
with stiff concrete or a cement gun, if 
available. 

For bulkheads over 3 ft thick, build- 
ing retaining bulkheads for forms is the 
best approach. Retaining bulkheads can 
be constructed from concrete, concrete 
block, or brick, but the latter two are 
preferred to forego the need of con- 
structing wood forms for the concrete. 
Mortar for block and brick must be prop- 
erly mixed to ensure good bonding. 

Concrete for the center plug can be 
mixed and placed underground by machine 
or from the surface through vertical 
borehole(s). Hand mixing and placing 
should not be attempted, owing to the 
large volumes involved. As mentioned 
earlier, the coarse aggregate can be pre- 
placed between the retaining bulkheads 
and concrete grout injected through the 
outby side of the bulkhead or from the 
surface through vertical boreholes. 



15 



Plywood, 4 -in minimum 
thickness 




Not to scale - --r^^ 

Steel reinforcing rods -*\ 

FIGURE 6. - Concrete being mixed underground and placed in forms by concrete pump. 




Not to scale 



FIGURE 7. - Pumping concrete from the surface 
through a single vertical borehole to a central un- 
derground site. 




Not to scale 



FIGURE 8. - Pumping concrete from the surface 
through a vertical borehole directly to bulkhead 
site. 



PRESSURE GROUTING ( 17 ) 

Pressure grouting involves the injec- surrounding inundation bulkheads is a 
tion of fluid materials, under pres- most significant factor in their long- 
sure, into rock or soil to fill pore terra performance, because it reduces 
spaces, consolidate material, and prevent strata permeability and increases 
water migration. Grouting the strata strength and durability with respect to 



16 



aqueous solutions. There are many fac- 
tors to consider in planning a grouting 
program, such as the drilling, spacing, 
and depth of holes; the proper selection 
of grouting materials and equipment; the 
control of grout volumes and injection 
pressures; and a knowledge of the strata 
to be grouted. Pressure grouting is a 
highly specialized technique requiring 
experience and sound engineering judg- 
ment, so that procuring the services of 
qualified personnel is essential. 

In some respect, pressure grouting is 
an art, for which the establishment of 
rigid rules and procedures is not fea- 
sible. However, a knowledge of basic 
grouting materials and methods is 
recommended. 

GROUTING MATERIALS 

An important factor in the successful 
grouting of permeable coals and other 
stratified rock is the selection of a 
suitable grouting material. There are 
four basic types of grout: Portland ce- 
ment, asphalt, clay, and chemical grouts. 
Technical literature and field experience 
show that Portland cement and chemical 
grouts are the most applicable and effec- 
tive for grouting coal mine strata. 

Portland Cement Grout 

Portland cement is the most widely used 
grouting material because of cost, avail- 
ability, and everyday knowledge of the 
material. Neat cement grout consists of 
Portland cement and water, but mineral 
admixtures are often used with this base 
to attain grouts with specific character- 
istics. There are five types of Portland 
cement (excluding air-entrailed cements) 
that conform to ASTM Designation CI 50 and 
can be used for cement grouts. Each type 
possesses specific properties that may be 
needed to meet job requirements. 

Type I is a general-purpose cement 
suitable for most grouting jobs. It is 
used when the special properties of the 
other four types are not needed. Type II 
has moderate resistance to the sulfates 
in groundwater. With type II the heat 
of hydration is less and develops at a 
slower rate than that of type I. Type 



III is used when early strength gains are 
required within 10 days or less. It also 
has a finer grind, which improves its in- 
jectability. Type IV generates less heat 
than type II and develops strength at a 
very slow rate. It is rarely used in 
grouting. Type V has a high resistance 
to sulfates and is used when groundwater 
with extremely high sulfate content is 
encountered. 

Mineral admixtures are finely divided 
materials that are added to neat cement 
grout to improve or achieve a specific 
characteristic. Calcium chloride, sodium 
silicate, and gypsum, when used in small 
amounts (2 to 4 wt pet) , act as accelera- 
tors and decrease the setting time of the 
grout. Accelerators are used when there 
is little heat to aid in setting. They 
may also be used to reduce grout migra- 
tion, reduce erosion of new grout by 
groundwater, and increase the rate of 
early strength gains. When high tempera- 
tures are encountered, retarders such as 
sodium chloride and calcium lignosulfo- 
nate are used to increase setting time. 
These admixtures allow the grout to mi- 
grate properly into fine pore spaces be- 
fore setting. 

Fly ash and natural pozzolans such as 
diatomite and pumicite are admixtures 
that when used in small amounts improve 
the pumpability of the mix. They may al- 
so be used as a filler and can compose up 
to 30 wt pet of the mix. In this case, 
they react chemically with the cement to 
produce cementitious properties and im- 
proved bonding. Other admixtures include 
bentonite, which is used to increase 
water requirements and reduce the unit 
weight of the mix; latex additives, which 
improve bonding and increase grout re- 
sistance to acids and other corrosive 
solutions; and aluminum powder, which in- 
creases viscosity and causes the grout to 
expand slightly. 

Water-to-cement ratios for Portland 
cement grouts are indicated by either 
weight or volume. The volume method is 
more convenient and most frequently used 
in the field. A sack of cement is con- 
sidered to equal 1 ft 3 . The mixing 
ratios of water to cement used most 
frequently range from 1:1 to 4:1. The 
choice of a starting mix depends on such 



17 



factors as the size and amount of pore 
spaces in the strata, the amount of wa- 
ter the strata bears and experience with 
grouting similar strata. In general, 
grouting is started with a thin mix. 
Thicker mixes are used based on the abil- 
ity of the strata to accept the grout. 
If the strata accepts the starting mix 
readily without pressure buildup, thicker 
mixes are considered in accordance with 
the objectives of the grouting program. 

Chemical Grouts 

In recent years, the use of chemical 
grouts to consolidate permeable rock and 
soil has gained increased popularity. 
The primary advantages of chemical grouts 
over Portland cement grouts are their im- 
proved bonding characteristics, low vis- 
cosity, better flowability, and good con- 
trol of setting time. Some chemical 
grouts are water reactive and expand 
slightly by contact with water, a feature 
that is advantageous in sealing fine pore 
spaces in the rock. Some disadvantages 
include possible toxicity, so that they 
may not meet MSHA permissibility stan- 
dards for use underground. Also, they 
are relatively higher in cost than Port- 
land cement grouts. 

Research and development is continuing 
at a rapid pace, and currently a number 
of commercial manufacturers produce chem- 
ical grouts and injection equipment. 
Most grouts consist of two or more compo- 
nents that must be mixed before injec- 
tion. Because of the critical nature of 
proportioning, this mixing should only be 
done under the supervision of company 
personnel. It is beyond the scope of 
this report to review all commercial 
grouts currently available. If one is 
considering the use of a chemical grout, 
the best approach is to consult directly 
with a company that has a proven grouting 
technique. 

GROUTING METHODS 

Procedures for grouting permeable 
strata vary, as dictated by the charac- 
teristics of the strata and the program 
objectives. Regardless of how much ex- 
ploratory drilling and other pregrouting 



investigation is done, the size and con- 
tinuity of pore space in the strata will 
remain relatively unknown. The art of 
successful grouting requires the ability 
to treat these unknowns through experi- 
ence obtained from similar grouting work. 
Three basic grouting methods are used in- 
mine to seal and consolidate permeable 
strata surrounding a bulkhead: curtain 
grouting, blanket grouting, and contact 
grouting. 

Curtain grouting involves the construc- 
tion of a curtain or barrier of grout by 
drilling and grouting a linear sequence 
of holes. Its primary purpose is to re- 
duce strata permeability. A grout cur- 
tain can consist of a single row of holes 
or two or more parallel rows. "Primary" 
holes are initially drilled into the 
roof, ribs, or floor on rather widely 
spaced centers ranging from 20 to 40 ft. 
After the two primary holes have been 
grouted, a first intermediate hole is 
drilled midway between them. After this 
hole is grouted, two secondary interme- 
diate holes are drilled midway between 
the primary and first intermediate hole. 
This pattern of drilling and grouting 
continues until grout consumption indi- 
cates the strata to be sufficiently 
tight. Grout consumption should decrease 
as the spacing of intermediate holes be- 
come smaller. 

The hole depth for curtain grouting de- 
pends on the flowability of the grout , 
the ability of the strata to accept 
grout, and the distance the grout must 
migrate to create a satisfactory seal. 
Generally, the primary holes are the 
deepest, with intermediate holes being 
drilled less deep with each successive 
grouting. 

Blanket grouting involves the injec- 
tion of grout, under low pressures, into 
shallow holes drilled on a grid pattern. 
Its primary purpose is to increase the 
bearing strength of the strata. Blanket 
grouting may be used to form a grout cap 
prior to curtain grouting and serve as a 
barrier to improve the migration of high- 
er pressure grout into deeper horizons, 
but it is more commonly used to consol- 
idate fractured or severely weathered 
strata in a mine entry prior to bulk- 
head construction. This grouting method 



\ 



18 



strengthens the strata and provides 
bearing support when constructing a large 
plug or excavating trenches to recess a 
bulkhead. Holes are drilled on 5- to 7- 
ft centers and are shallow, 3 to 5 ft 
deep. Severely fractured strata may re- 
quire the holes to be drilled on tighter 
spacings of 1 to 3 ft. 

Contact grouting involves the grouting 
of the voids between the roof, ribs, and 
floor of the entry and the bulkhead or 
plug. These voids result primarily from 
improper concrete placement and concrete 
shrinkage while curing. This is consid- 
ered a most important grouting procedure, 
because it improves bonding and prevents 



water seepage along this concrete-strata 
interface. Over the long term, it mini- 
mizes the premature failure of bulkhead 
anchorage. 

Holes for contact grouting are usually 
provided for by placing steel pipe or 
packers at predetermined locations along 
the concrete-strata interface before the 
concrete is poured. The pipes, which 
protrude from the forms , act as a travel- 
way for the grout after the concrete 
cures. At times, during the pouring of 
the plug, the pipe may fill with concrete 
which must then be drilled out so that 
grout can migrate properly along the 
interface. 



BARRIER PILLARS 



PILLAR CONSIDERATIONS 

Bulkheads can be designed to withstand 
a considerable amount of hydrostatic 
pressure, but the seal is only a small 
part of the water impoundment. The pe- 
rimeter of the abandoned area, consisting 
of chain or barrier pillars, forms a 
large part of the impoundment and at 
times may not be capable of withstanding 
the design pressure. Practical limits of 
inundation can be determined by plotting 
on a coal contour map the expected mine 
pool elevations and corresponding ground 
surface elevations. Areas of excessive 
pressure are projected, and determina- 
tions are made as to the capability of 
the coal pillars to withstand the antici- 
pated water pressure. 

There are no specific Federal regula- 
tions concerning the size of barrier pil- 
lars that separate active mines from 
inundated abandoned areas. In general, 
there are two ways MSHA handles potential 
inundations involving barrier pillars. 
Their first concern is whether the situa- 
tion presents an imminent danger to the 
mine and the workers. Second, if no im- 
minent danger exists, is whether proper 
procedures (such as drilling, etc.) are 
being followed when mining toward or 
adjacent to impounded water. 11 A number 
of questions are considered to determine 
if an imminent danger exists. First, 
does the coal barrier have an adequate 
width? Second, what is the hydrostatic 



head and the amount of water impounded? 
Third, what is the physical condition of 
the barrier? Fourth, if the barrier were 
to fail, is there sufficient time to warn 
and evacuate workers (18)? 

In most cases, before an underground 
impoundment is created by constructing 
bulkheads, the width of the coal pillars 
is known. Determinations must then be 
made as to the limits of hydrostatic 
pressure that the pillars can withstand. 

PILLAR WIDTH FORMULAS ( 18-2 1 ) 

Determining whether an existing coal 
pillar is sufficiently wide to resist a 
specific waterhead is a complex problem 
that cannot be solved with a high degree 
of certainty. However, several formulas, 
based on experience and empirical obser- 
vation, have been developed for this 
purpose. 

The first is the Ashley, or Mine In- 
spector, Formula, established by a seven- 
member commission for the Commonwealth of 
Pennsylvania for incorporation into State 
law. The primary objective of the com- 
mission was to develop a method of 

11 Section 107(a) of the 1977 Act covers 
the imminent danger situation and Sec- 
tions 75.1200, 1200-1, 1200-2, 1201, 
1202, 1202-1, 1203, 1204, and 1701 of 
Title 30, Code of Federal Regulations, 
outline the criteria governing MSHA's en- 
forcement activities. 



19 



designing coal barriers to impound water 
and protect active mines from unexpected 
inundations. From the findings of the 
commission, the minimum width of the bar- 
rier is expressed as 



W = 20+4T+0.1D 



(7) 



where W = Width of the coal pillar, ft, 

T = average thickness of the coal 
seam, ft, 

and D = depth of overburden or the 
height of waterhead, ft. 

Knowing W and T, the maximum waterhead 
that a barrier can withstand (D) can be 
determined. 

A second formula, developed in England 
through observation and measurement, is 
based on the pressure arch concept of 
stress distribution. Here the width of 
the barrier is presumed to be 



W = 0.15D+60 



(8) 



where W = Width of the coal pillar, ft, 
and D = depth of overburden, ft. 

This formula does not take into account 
the thickness of the seam and consequent- 
ly may be unsatisfactory for water im- 
poundments. Data and field experience 
presented during the development of the 
Ashley Formula indicate that seam thick- 
ness is an important factor in the design 
of coal barriers. With all other factors 
being equal, a thick seam requires a 
thicker barrier than a thin seam. 

The third formula, developed by C. T. 
Holland (20) , is the least used of the 
three. It has been stated that this for- 
mula is not suitable for computing water 
dams , although it has been compared with 
the two previous formulas ( 21 ) . The 
width of the barrier is given as the 
greater of 



W = 15T or 5 



log 2 W 2 
0.09 log I 



(9) 



where 



W = Width of the coal pillar, 
ft, 



T = average thickness 
coal seam, ft, 



of the 



W 2 = the estimated convergence on 
the high stress side of the 
pillar, mm, 

ji. = the base of the natural sys- 
tem of logarithm (2.72), 

5 = a constant which includes a 
factor to convert metric to 
English units and a safety 
factor; 

and 0.09 = a coefficient if caving fol- 
lowing mining is permitted. 

Holland suggests that the convergence 
factor, W 2 , be estimated as a function of 
overburden depth according to 



W 2 = 10 10.0012D 
where W 2 = Convergence, mm, 
I = 2.72, 



(10) 



and 



D = depth of overburden, ft. 



This relationship gives the convergence 
at various depths for a coal bed 7 ft 
thick and having a strength of 3,000 psi 
in a 3-in cube. 1 2 it should be noted 
that under the assumed conditions, the 
Holland Formula will give wider barriers 
than the Ashley Formula at all depths. 

The actual safety factor associated 
with these formulas cannot be readily de- 
termined because of the many elements 
that have an unknown effect on the barri- 
er. These include stress redistribution 
after mining, subsidence, geologic fea- 
tures such as slips and faults , the long- 
term effects of water seepage, pore water 
pressure, ground saturation, and dete- 
rioration and the favorable aspects of 
pressure grouting. Since no one formula 
is universally applicable, mine personnel 
must exercise sound engineering judgment 

' ^To estimate convergence for coals 
having different seam thickness and 
strengths, see Holland (20). 



20 



in determining the practical limits 
of potential inundation. The design 
of bulkheads for underground water 



impoundments should be based on the max- 
imum hydrostatic pressure that a coal 
barrier can withstand. 



MONITORING WATER PRESSURE 



The hydrostatic pressure in the inun- 
dated area should be monitored. There 
are two basic methods to accomplish this: 

(1) from vertical boreholes drilled from 
the surface to the inundated area, and 

(2) in-mine, through piping cast into the 
bulkheads themselves. The first method 
involves the drilling of a vertical bore- 
hole from the surface to the inundated 
area and measuring the height or pressure 
of a column of water with a water-level 
indicator or pressure transmitter. These 
instruments are commercially available 
from manufacturers of geophysical and hy- 
drological instrumentation. 

One type consists of a detection meter 
and a water-sensitive electrode attached 
to 300 to 500 ft of electrical cable num- 
bered in 1- to 5-ft intervals. The level 
of the water below the surface is mea- 
sured by lowering the electrode down the 
borehole until a sharp needle deflection 
on the meter indicates that water is con- 
tacted. The approximate waterhead is de- 
termined by subtracting this distance 
from the total depth to the coalbed where 
the bulkhead(s) is(are) located. To con- 
vert waterhead expressed in feet to hy- 
drostatic pressure in pounds per square 
inch, multiply by 0.434. 

Another system uses a float to measure 
the water column in the monitor wells. 
The float is connected to a steel wire 
upon which are crimped brass beads at 
6-in intervals, to prevent line slippage. 
The line wraps around a measuring wheel 
and an idler pulley, and the wheel move- 
ment drives a depth recorder. On the 
older mechanical recorders, the wheel 
drove a pen directly, and a spring or 
electric motor ran a time drive. The 
spring drives can operate for up to 6 
months without winding, and the chart pa- 
per comes in rolls good for up to 2 yr, 
both depending upon drive speed. Newer 
recorders have been especially designed 
for telephone transfer of the data, and 
an add-on device is available to con- 
vert the older mechanical recorders to 



transmitting recorders. The floats come 
in various sizes, with a 3-in-diam float 
presently being the smallest. Because 
the float must be counterweighted and the 
excess line must hang in the hole, the 
smallest practical hole size is 4 in ID, 
although larger sizes are recommended to 
allow the float and counterweight to pass 
each other without interference. The 
float-beaded line systems have been used 
in holes as deep as 650 ft, with float 
depths up to 350 ft, with no difficulty. 

Still another method of monitoring 
pressure from vertical holes is to in- 
stall strain-gauge type pressure trans- 
mitters. Pressure transmitters capable 
of operation in water at depths as great 
as 5,000 ft are readily available. Al- 
though the difficulties of finding drift- 
free transducers for long-term installa- 
tion and connecting them to a cable are 
not trivial, a few manufacturers have 
partial systems available. Pressure 
transmitter systems are also available 
from companies serving oilfield needs. 
Most of these are strain-gauge type sys- 
tems, although one company makes a system 
which uses a small-diameter tube and a 
gas-filled chamber at the bottom of the 
hole with the pressure transducer at the 
surface. Both the strain-gauge systems 
and the gas chamber system are capable of 
operating under high pressures. However, 
these systems are very expensive and many 
are available for rental only, and on a 
short-term basis. 

A second and more direct method of mon- 
itoring hydrostatic pressure involves 
casting a pipe into the bulkhead and in- 
stalling a pressure gauge or pressure 
transducer. Pipes can be either plastic 
or metal, though metal pipes must be cor- 
rosion resistant. Care must also be tak- 
en if plastic pipe is used to insure that 
the pipe will be able to withstand the 
maximum anticipated hydrostatic pressures 
(this of course applies to all valves and 
fittings). Pipes should be installed 6 
to 12 in from the floor and 12 to 18 in 



21 



from the rib having the lowest elevation. 
The pipe should have a 1- to 2-in ID and 
extend 2 to 3 ft from either side of the 
bulkhead. Both ends of the pipe must be 
threaded to permit the installation of a 
piezometer installed on the inby side of 
the bulkhead (water side) and a pressure 
gauge or pressure transducer and valve on 
the outby side. The piezometer allows 
water to pass into the pipe, but traps 
sediments that can clog the pressure 
gauge and affect readings. Piezometers 
can be handmade from a porous , fine- 
grained material such as sandstone, but a 
more preferable type is constructed from 
porous polyethylene. This type is avail- 
able commercially; it is lightweight and 
relatively inexpensive. 13 

It may often be desired to allow remote 
reading of pressures from behind the 
bulkheads. Strain-gauge pressure trans- 
ducers for this use are readily avail- 
able. Unlike the case of the vertical 
boreholes where the transducer (or trans- 
mitter) case and cable connector must 
withstand hydrostatic pressure and be 
leakproof , the in-mine transducers need 
only be intrinsically safe and be air- 
tight to prevent dust entry. Many pres- 
sure transducers are especially made for 
use in hazardous environments. 

Two types of transducers are available. 
Both may be obtained in the same sizes, 
and both require input voltages in the 
range of 6 to 60 V dc. The first type 
has a constant current output, usually in 
the range of 16 to 20 mA, and a fluctuat- 
ing voltage in the range of 0.5 to 5.0 V 
dc. The second type maintains a constant 
voltage, and the output signal is a 
changing current, usually varying between 
4 and 20 mA. The first type allows data 
transmission over a greater distance but 
requires higher power, and the second 
type is more likely to be available in 
intrinsically safe models. 

The cases of the pressure transducers 
come in a variety of materials, the most 
common being stainless steel (several 

1 3 For more information regarding poly- 
ethylene piezometers, see "Installation 
of Piping" in appendix A. 



types) and titanium. They can be made 
with a number of thread types so that the 
gauges can be screwed directly into a 
pipe cast into the bulkhead. 

The pressure gauges and pressure trans- 
ducers must be chosen such that their 
range is greater than the maximum hydro- 
static pressure that can develop behind 
the bulkhead; they should be calibrated 
before installation. A valve should also 
be included in the line between the bulk- 
head and any pressure transducer or gauge 
to allow their removal for replacement or 
calibration. Usually the valve is kept 
shut for direct reading gauges , except 
when the gauges are being read. When 
readings are made the valve should be 
opened slowly to prevent shock damage to 
the gauge. 

Finally, it may be necessary In some 
cases to have a warning system, should an 
inundation occur by bulkhead or barrier 
pillar failure. One method of accom- 
plishing this is to install remote read- 
ing water-level warning devices in the 
areas of interest. A number of on-off 
type sensors are available for this use. 
Most of these are float-level switches 
and indicators. These devices come in a 
wide variety of configurations and power 
ranges. Many of them are also designed 
to actuate equipment , such as alarms , 
pumps, or motors. The problem with these 
devices is that most have moving parts 
that may be frozen by dust or corrosion. 

Another device, which has no moving 
parts, is the capacitive proximity sen- 
sor. These sensors are only suitable for 
sensing, in a yes-no fashion, the pres- 
ence or absence of water, but they re- 
quire little power and have no moving 
parts. Typical capacitive proximity sen- 
sors operate on 10 to 12 V dc at currents 
of 5 to 20 mA. They are not actuated by 
a moisture film on the sensor head and 
are not affected by moderate quantities 
of dust. 

To insure the safety of the mine and 
the workers, hydrostatic pressure must 
not exceed the capacity of the bulk- 
head(s) or coal barrier(s). If dangerous 
pressures are suspected, ways of reducing 
excessive pressure should be implemented. 



22 



This can be accomplished by pumping the 
water out from the surface or draining 



the area behind the bulkhead 
in the active working areas. 



into sumps 



OBTAINING MSHA APPROVAL 



Creating an underground water impound- 
ment by constructing bulkheads requires 
the approval of the MSHA District Mana- 
ger. Federal regulations governing min- 
ing activities near water bodies, the 
construction and inspection of bulkheads, 
the width of coal barriers needed, and 
MSHA' s enforcement role are covered in 
Title 30, CFR, Part 75. State regula- 
tions may be more specific and stringent, 



and mine operators should familiarize 
themselves with the State law. Initial- 
ly, the mine operator is required to pre- 
pare detailed plans on bulkhead design, 
construction, location, etc. , and submit 
them to the appropriate MSHA district of- 
fice for approval. If disapproved, areas 
of insufficiency are defined and the op- 
erator must make changes as recommended. 



DISCUSSION 



The practice of constructing bulkheads 
in underground coal mines for the purpose 
of impounding water will become increas- 
ingly important in future years. While 
most of the practical experience of de- 
signing and constructing bulkheads has 
been in hard-rock mining, it can be con- 
cluded that no stringent guidelines or 
theoretical design criteria have been 
widely accepted. Bulkhead design can 
differ depending on the condition of the 
surrounding rock and the anticipated hy- 
drostatic pressure. Over-design is a 
common practice. Bulkheads are usually 
constructed sufficiently thick to resist 
the force of hydrostatic pressure and the 
surrounding strata pressure grouted to 
minimize water seepage. 

Little reference has been made in this 
report to the safety factors associated 
with the designs discussed. Such safety 
factors are difficult to assess because 
of the many variables that can affect 
bulkhead design. These include the maxi- 
mum waterhead the bulkhead is designed 
to withstand; the type of anchorage; the 
strength and condition of the anchoring 
strata; the existence of geologic anoma- 
lies in the immediate area; the favorable 
aspects of pressure grouting; and the 
maximum pressure that the barrier pillars 
can safely withstand. However, accept- 
able margins of safety can be made in- 
herent to these designs if one selects 



conservative strength values for rock and 
concrete in the design equations. It is 
not an uncommon practice to increase re- 
quired bulkhead thickness by a factor of 
1.5 to 2, because a large safety factor 
far outweighs most criteria in the theory 
of design. 

The success of any underground impound- 
ment depends on the ability of the entire 
dam structure, consisting of the bulk- 
heads and barrier pillars, to withstand 
the anticipated hydrostatic pressure. In 
some instances, the barrier pillars may 
form the weakest link in the impoundment 
and therefore, dictate the feasibility 
and practical limits of potential inunda- 
tion. This is an important consideration 
because with time the barrier pillar can 
be just as prone to failure as the bulk- 
head. The deterioration of coal barriers 
and surrounding roof and floor strata by 
water seepage can be minimized through 
pressure grouting. 

Creating an underground water impound- 
ment also creates the potential for an 
inundation hazard. Mine personnel must 
exercise sound engineering judgment in 
design. To insure the safety of the 
workers and the mine, water levels should 
be monitored and controlled, ground move- 
ments near the impoundment area stabi- 
lized with supplemental supports, and the 
bulkheads and barrier pillars inspected 
regularly. 



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23 



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24 



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Flatau, A. S., R. W. Brockett, and 
J. V. Brown. Grouts and Grouting: Sur- 
vey of Materials and Practice. Civ. Eng. 
and Pub. Works Rev., v. 68, No. 804, 
1973, pp. 591-601. 



25 



Frailey, D. Why, When and How to Seal 
Abandoned Workings Rather Than Ventilate. 
Trans. Nat. Safety Congr. , v. 7, 1968, 
pp. 21-22. 

Garcia, J. A., and S. M. Cassidy. 
Bulkheads for Coal Mines. AIME Tech. 
Pub. 789, 1937, 17 pp. 



Pugh, W. L. Water Problems at West 
Cannock No. 5 Colliery. Min. Eng. (Lon- 
don), Feb. 1980, pp. 669-679. 

Skelly and Loy. Guidelines for Mining 
Near Water Bodies (contract H0252083) . 
BuMines OFR 29-77, 1976, 107 pp.; NTIS PB 
264-728/AS. 



Graham, R. W. A Discussion of Pressure 
Grouting. Q. CO Sch. Mines, v. 58, No. 
4, Oct. 1963, pp. 129-136. 

Matthes, G. How Engineers Beat Shaft 
Flood in Karst Limestone. World Min. , v. 
28, No. 2, 1975, pp. 48-52. 

Miller, J. T. , and D. R. Thompson. 
Seepage and Mine Barrier Width. Paper in 
Fifth Symposium on Coal Mine Drainage Re- 
search (Proc. Coal and the Environment 
Tech. Conf., Louisville, KY, Oct. 22-24, 
1974). Nat. Coal Assoc, 1974, pp. 103- 
127. 

Mitchell, R. J., J. D. Smith, and 
D. J. Libby. Bulkhead Pressure Due to 
Cemented Hydraulic Mine Backfills. Can. 
Geotech. J., v. 12, No. 3, 1975, pp. 362- 
371. 

Nishimatsa, Y. , Y. Oka, and Y. Nishida. 
The Failure and Displacement of Concrete 
Plug for Sealing in the Pit Mouth of 
Closed Mine. Rock Mechanics in Japan 
(Engl. Transl.), v. 3, 1979, pp. 114- 
116. 

Orchard, R. J. Working Under Bodies of 
Water. Min. Eng. (London), v. 134, No. 
166, Mar. 1975, pp. 261-270. 

Pearson, M. L. , T. Preber, and P. J. 
Conroy. Outcrop Barrier Design Guide- 
lines for Appalachian Coal Mines (con- 
tract J0395069, Dames & Moore). BuMines 
OFR 134-81, Feb. 1981, 166 pp. 

Polatty, J. M. Comments on U.S. Grout- 
ing Practices. Paper in Foundation for 
Dams (Proc. Eng. Foundation Conf., Pacif- 
ic Grove, CA, Mar. 17-21, 1974). ASCE, 
1974, pp. 47-55. 



South African Council for Scientific 
and Industrial Research, National Mechan- 
ical Engineering Research Institute. A 
Survey of Factors Which Have a Bearing 
Upon the Design of Underground Plugs. 
CSIR Rep. ME/MR/4267, 1963, 4 pp. 

. A Survey of Underground Plugs 

Installed in South African Gold Fields. 
Contract Rep. C MEG/576, Nov. 1963; Cham- 
ber of Mines Res. Rep. 30/64, Aug. 1964, 
42 pp. 

Tillson, B. F. Pump Stations and Water 
Control: Bulkheads, Dams Ejectors, Prim- 
ing, Siphons, Sumps. Ch. in Mine Plant. 
AIME, 1938, pp. 172-181. 

Venburgh, L. C, D. Sokol, and M. V. 
Damm. Investigative Programs for Design- 
ing and Modeling Mine Water Control Sys- 
tems. Min. Eng. (London), Aug. 1982, 
pp. 1217-1219. 

Walker , J. Grouting Techniques . Plant 
Eng., v. 11, No. 5, 1967, pp. 321-325. 

Whittaker, B. H. , and R. N. Singh. De- 
sign Aspects of Barrier Pillars Against 
Water-Logged Workings in Coal Mining Op- 
erations. Paper in Symposium on Water in 
Mining Underground Works. Assoc. Nac. de 
Ing. de Minas, Granada, Spain, 1978, v. 

1, pp. 675-692. 

Wittke, W. Application of the Finite 
Element Method to the Design of Under- 
ground Bulkheads. Erzmetall, v. 26, No. 

2, 1973, pp. 66-74. 

World Mining. How West Driefontein 
Gold Mine Fought and Won the Flood Bat- 
tle. V. 23, Mar. 1969, pp. 38-43. 



26 



APPENDIX A. —HYDROSTATIC TESTING OF A SINGLE BULKHEAD SEAL 



BULKHEAD CONSTRUCTION 



The general diagram of the bulkhead is 
shown in figure A-l. It was constructed 
from 6- by 8- by 16-in solid concrete 
block and was located in a 6- by 18- by 
20-ft dead-end room of the Bureau's Safe- 
ty Research Coal Mine, as shown in figure 
A-2. The first task was to trench the 
ribs and floor to provide anchorage for 
the bulkhead. The roof was not trenched 
because this was considered too hazard- 
ous. Using an air-driven jack hammer and 
chisel, trenches were dug 16 in into each 
rib and 22 in into the floor. Care was 
taken during this operation so that the 
strata were not unnecessarily cracked or 
fractured. When the trenching was com- 
pleted, debris was removed from the floor 



trench and a level concrete footer ap- 
proximately 4 in thick was poured. After 
allowing the footer to set, construction 
of the bulkhead began, as shown in figure 
A-3. 

To facilitate the installation of an 
air release, water inlet, and pressure 
gauge, piping had to be built into 
the bulkhead. Special blocks were made 
by drilling 1-1/4-in holes lengthwise 
through the block and grouting a 1-in 
steel pipe into place, as shown in figure 
A-4. The blocks were then laid in their 
respective courses as the bulkhead was 
constructed, as shown in figure A-3. 

As the bulkhead approached roof level, 
gunite was sprayed in the cavity behind 



Pilaster center 



Transverse pattern 
of laying block 




( Not to scale 



FIGURE A-l. - Diagram of single bulkhead seal 



27 



\ 



Concrete block bulkhead 



J"~L 




F- Butt 











X 





E-Butt 



D-Butt 



C-Butt 

i_/ u — 






L 



50 

i 



100 



Scale, ft 

FIGURE A-2. - Location of bulkhead in Safety Research Coal Mine. 



the bulkhead, approximately 1/2 in to 1 
in thick, to seal cracks in the roof, 
ribs, and floor and provide a watertight 
chamber. The bulkhead was then completed 
to roof level and sealed tight against 
the roof using 2- by 8- by 16-in solid 
block and mortar. Roof anchorage was 
provided by securing 4- by 4-in angle 
irons on either side of the bulkhead ' s 



pilaster on both the inby and outby 
sides. Eighteen-inch mechanical roof 
bolts were used in securing the angle 
irons to the roof. The bulkhead cured 
for a week before gunite was sprayed, 1/2 
in to 1 in thick, on the outby side. Af- 
ter spraying, the bulkhead was allowed 
to cure for another week before testing 
began. 



TEST APPARATUS 



Figure A-5 shows a diagram of the test 
area with the following apparatus: 

1. Water pipe - inlet for water. 

2. Air release - allows air to escape 
as the chamber is filled with water. 



3. Standpipe - a clear plexiglass tube 
that indicates water level behind the 
bulkhead . 



4. Pressure gauge - measures 
static pressure behind bulkhead. 



hydro- 



28 




FIGURE A-3. - Bulkhead under construction in Safety Research Coal Mine. 



5. Water meter - measures amount of 
water used during testing. 



7. Water tank - water storage for 
test. 



6. Pump - air-driven pump with con- 
trols to regulate pressure behind 
bulkhead. 

TEST PROCEDURE 



The test procedure consisted of incre- 
mentally building and then relieving hy- 
drostatic pressure behind the bulkhead. 
This procedure simulated the actual in- 
mine practice of allowing hydrostatic 
pressure to build to a maximum level, and 
then relieving it by pumping or draining 
excess water behind the bulkhead. Ini- 
tial pressure was started at zero and 
increased at 5-psi intervals. Each 



increment was maintained for approximate- 
ly 10 min. Pressure was then dropped 
back to zero and a new series of tests 
started, as shown in table A-l. 

The bulkhead was inspected after each 
series of tests. The decision to proceed 
to the next test depended on several fac- 
tors: (1) success of the previous test; 
(2) excessive water leakage or damage to 
roof, ribs, or floor strata; (3) visible 



29 



74- in steel plate 
(welded to pipe) 



l-in 
steel pipe 




Grout 



FIGURE A-4. 



Scale, in 

Cross section of l-in pipe grouted into concrete block. 



TABLE A-l. - Test procedure for 
incrementally pressurizing 
bulkhead 



Test 


Date 


Pressure 


Time, 






interval, ' psi 


min 


&•••••••• 


4/28/82 


0- 5 


10 




4/28/82 


0-10 


20 




4/28/82 


0-15 


30 


^•♦•••••i 


4/29/82 


0-20 


40 




5/ 3/82 


0-25 


50 




5/ 3/82 


0-30 


60 




5/ 3/82 


0-35 


70 




5/ 4/82 


0-40 


80 




5/ 4/82 


0-45 


90 




5/ 5/82 


0-50 


100 



'5-psi intervals. 

water leakage or damage to the bulkhead 
structure. 

When testing commenced, water immedi- 
ately began to leak from the roof, ribs, 
and floor strata surrounding the bulk- 
head. During the fourth series of tests 
at 20 psi pressure, water leakage became 
excessive, but there was still no leaking 
through the bulkhead itself. Apparently, 
the gunite did not make a good sealant 
and the pressure was forcing the water 
through the gunite and into the strata. 
Testing was temporarily discontinued for 




(Not to scole) 

FIGURE A-5. - Diagram of test apparatus. 

fear that infusing the strata with water 
would create larger cracks and generally 
weaken ground conditions in the test 
area. To seal and strengthen the sur- 
rounding strata, the application of a 
pressure grouting technique was consid- 
ered the most practical approach. 



30 



PRESSURE GROUTING 



Pressure grouting was considered the 
most practical method of sealing the 
strata in the test area. The strategy 
was to stop water permeation by forming 
a grout curtain around the water chamber 
behind the bulkhead. In addition to 
minimizing seepage, the grout would 
strengthen the ground by consolidating 
the fractured strata. The Bureau sought 
a company with expertise in a proven 
pressure grouting technique to do the 
work. Representatives of Mobay Chemical 
Corp. were contacted; after consulta- 
tion, the decision to inject their Roklok 
B-4 waterstop system into the fractured 
strata was made. 

The B-4 system is a two-component poly- 
urethane grout consisting of a polymetric 
isocyanate (component A) and a polyol 
resin (component B) , which are mixed and 
injected into the strata. The mixture 
has a low viscosity, which enables it to 
flow freely into fine cracks and fis- 
sures. As it migrates into the strata, 
it encounters water and expands driving 
any remaining water out. The component 
mixture solidifies in approximately 5 
min. 

The major concern was grout migration 
to the blind face located behind the 
bulkhead, because it was suspected that 
most of the water was seeping through 
this area. A drilling plan for injecting 
the chemicals was developed. To reach 
the blind face behind the bulkhead, two 
holes over 50 ft long had to be drilled 
from entries to the right and left of the 
bulkhead. Other holes would be drilled 
around the bulkhead to seal the ribs , 
roof, and floor, as shown in figure A-6. 
A total of 14 holes, varying in length 
from 4 ft to 50 ft, were drilled and 
injected. 

Injecting the grout was relatively sim- 
ple. A hole was first drilled into the 
strata and the packer-mixer assembly, 
with an expansion shell, was inserted 
into the borehole and anchored tightly, 
as shown in figure A-7. This assem- 
bly was then connected to the pumping 
unit by high-pressure hoses. The two 
components were pumped separately, then 
mixed and injected into the strata via 



the mixer-packer assembly. The grout was 
pumped continuously, under pressure, as 
it migrated into the strata. Injection 
pressures ranged from 500 to 700 psi. 
When the grout emerged from the strata, 
as shown in figure A-8, the injection was 
stopped, the hole abandoned, and a new 
hole was drilled and injected. The in- 
jection started with hole Hj and pro- 
ceeded in numerical order to hole H 14 . 

Some roof sag was experienced when the 
grout was injected into the roof. To 
provide additional support, four timbers 
on 4-ft centers were installed across 
the entry, approximately 5 ft from the 
bulkhead. Extensometers , which detect 
roof sag, were installed and monitored 

TOP VIEW 




u+ l^ 



] Water chamber ; 




v / j r fh r\ y * - < { - re' 



FRONT VIEW 



\H 5 




r 


:./ 


He 


/H 9 




_ 


i 










■ 


1 






L 


I 1 




Rib i- 


1 1 


Rib 




f*" 


r iii 














r?rS 









H (0 H ( | H| 2 
SECTION A-A' 




Hole 


Depth, ft 


Hi 


4 


H? 


4 


H* 


15 


H„ 


15 


H S 


6 


H 6 


10 


H 7 


6 


Hfl 


10 


H 9 


6 


Hio 


6 


Hi, 


6 


H| 2 


6 


H-13 


52 


H|4 


52 



FIGURE A-6. - Drill plan for injecting polyure- 
thane grout in strata surrounding the bulkhead. 



31 




FIGURE A-7. - Packer-mixer assembly installed in borehole. 




FIGURE A-8. - Polyurethane grout emerging from strata. 



32 



continuously. Approximately 0.3 in of 
roof sag was measured during the roof 
injection, but no roof problems were 
encountered. 

Periodically during the injection, 
the bulkhead was pressurized at less 
than 20 psi to determine if the grout was 



migrating properly. These spot checks 
showed that the technique was working 
well. The pressure rose rapidly as water 
was pumped into the chamber behind the 
bulkhead and visible water seepage from 
the roof, ribs, and floor was signifi- 
cantly less. 



FINAL TESTS AND RESULTS 



With the pressure grouting completed, 
testing resumed and followed the origi- 
nal plan of incrementally pressurizing 
the bulkhead (see table A-l). Tests 1 
through 7 proceeded well; 35 psi (80 ft 
of waterhead) was reached with no signs 
of failure in the bulkhead or its anchor- 
age. Water seepage from the surround- 
ing strata was minimal, showing that the 
grout adequately sealed the water chamber 
behind the bulkhead. When 40 psi (92 ft 
of waterhead) was reached during test 8, 
the bulkhead began to show signs of fail- 
ure. Water was leaking, although not 
excessively, at roof level and also 
through mortar joints located near the 
base of the bulkhead. Testing proceeded 
to tests 9 and 10. With each pressure 
increase, water leakage through the roof 
and mortar joints became more severe. At 
50 psi (115 ft of waterhead) the bulkhead 
and roof strata were leaking excessively. 
It was at this point the bulkhead was 
considered to have failed and testing was 
stopped. Though testing continued to 50 
psi, the point at which the bulkhead 



initially showed signs of failure (40 
psi) was considered the maximum pressure 
the bulkhead could withstand. 

It should be noted that the hydrostatic 
pressures were applied over a much short- 
er time than under actual mine condi- 
tions. Infusing the strata with water 
was considered dangerous to ground sta- 
bility, limiting the time duration of 
tests. The maximum pressure the bulkhead 
withstood, 40 psi (92 ft of waterhead), 
includes no factor of safety, and actual 
pressures should be kept well below this 
limit because of this time factor. A 
flexural strength analysis of this bulk- 
head, given in appendix B; includes the 
maximum allowable stress a bulkhead of 
this type could safely withstand. 

Tests show that certain construction 
and maintenance procedures should be fol- 
lowed when building this particular bulk- 
head, especially if it is to act as both 
an explosionproof and water seal. These 
procedures are detailed in the next 
section. 



CONSTRUCTION AND MAINTENANCE PROCEDURES 



The following procedures for bulkhead 
construction and maintenance are based on 
test experiences and pertinent litera- 
ture. If implemented properly, they will 
decrease the possibility of an unexpected 
inundation and provide a greater degree 
of safety to the mine. 

CONSTRUCTION 

1. Construct the bulkhead in competent 
ground that is not excessively broken or 
fractured, preferably where ground move- 
ment has settled (l). 1 



2. Proper anchorage of the bulkhead 
to the mine roof, ribs, and floor is 
important and depends on strata type and 
condition. In general, most coals have 
good anchorage characteristics, and 
trench depths in the ribs of 16 to 24 in 
should be adequate ( 4^ . Fireclays, lime- 
stones, and shales, which compose most 
floor strata, usually make good support 

1 Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendixes. 



33 



material unless softened by water (_3) . 
Floor trenches should be at least 16 in 
deep if the strata are in good condition. 
If the floor is in poor condition, trench 
until competent strata are reached. 

3. If feasible, trenching of the roof 
is highly recommended. Due to roof sag, 
the roof is typically the place where 
most seepage occurs. Also, during tests, 
most water seepage occurred at roof level 
because the bulkhead was not keyed into 
the roof. To trench the roof, first re- 
move any headcoal or other deteriorat- 
ing roof rock so as to expose competent 
strata. Trenches should be made at least 
8 in deep , and care should be taken when 
trenching because of the unpredictable 
nature of most roof rock. 

4. Trenches can be cut with any hy- 
draulic or air-driven tools, continuous 
miners, or cutting machines. If extreme- 
ly hard strata are encountered, trenching 
may be carried out by drilling and shoot- 
ing, but care should be taken to avoid 
unneccessarily fracturing the strata (3) . 

5. Use only solid concrete block to 
construct the bulkhead. Remove all de- 
bris from the floor trench, and pour a 
level concrete footer on which to build. 
Mix cement properly to assure good bond- 
ing between blocks and make sure all 
courses are laid level. 

6. As the bulkhead is being built, 
fill in all gaps and cracks in the an- 
chorage with concrete or cement. When 
completed, use the same material to seal 
the bulkhead tightly against the roof , 
ribs, and floor (3). 

7. The bulkhead will need a water 
sealant to protect it from prematurely 
deteriorating but allow the bulkhead to 
cure for several days before application. 
In constructing the bulkhead for tests, 
gunite was used to seal the bulkhead. 
But, because gunite is sprayed on, air 
becomes trapped in the material; this may 
make it porous. To avoid this, use seal- 
ants that can be applied with a trowel 



such as a waterproofing cement. Apply 
several coats, and if possible seal both 
sides of the bulkhead. 

8. Pressure grouting is recommended 
depending on the permeability and 
strength of the surrounding strata. Of- 
ten, the deterioration of the roof, rib, 
and floor anchorage by water permeation 
creates a most significant structural 
concern. Tests showed that pressure 
grouting will minimize water permeation, 
especially if large pressures are antici- 
pated over the life of the bulkhead. If 
pressure grouting is necessary, the mine 
operator should carefully plan his strat- 
egies and select a company with a proven 
pressure grouting technique. 

MAINTENANCE 

1. The bulkhead should be inspected 
according to the Code of Federal Regula- 
tions, Title 30, Parts 75.303 and 75.305. 
Records of these inspections should be 
kept and include significant factors such 
as present waterhead, visible water seep- 
age through bulkhead or immediate strata, 
or any visible deterioration in the bulk- 
head or ground conditions. 

2. Ground movements, such as floor 
heave, roof convergence, and pillar 
spalling, can damage the bulkhead and its 
anchorage, especially if it is con- 
structed from rigid materials such as 
concrete or concrete block (J_) . Ground 
movements are difficult to predict, and 
in most mines they are inevitable. Be- 
cause of this , supplemental roof supports 
such as cribs and timbers should be in- 
stalled around the bulkhead as a routine 
measure. 

3. With time, water may weaken the 
bulkhead and anchorage, but this can be 
minimized by repatching and resealing. 
In extreme cases, where strata or bulk- 
head deterioration may endanger worker 
safety or the mine, more extensive mea- 
sures such as pressure grouting or recon- 
struction may be necessary. 



34 



4. Water pressure should not exceed 
the design capacity of the bulkhead. 
This capacity can vary, depending on such 
factors as construction practices, 
strength and permeability of anchoring 
strata, and ground stability. In addi- 
tion, large volumes of water stored 
behind a bulkhead (such as an open dam 



wall) can be just as dangerous as 
excessive water pressures. If dangerous 
water volumes are suspected, methods for 
reducing excess water should be imple- 
mented. This can include pumping the 
water out from the surface or draining 
the water from behind the bulkhead to un- 
derground sumps . 



INSTALLATION OF PIPING 



Pipes for monitoring water levels be- 
hind the bulkhead are recommended. The 
pipes can be either plastic or metal. If 
metal, they should be corrosion resist- 
ant. Installation of pipes, by grout- 
ing them into the concrete block, is dis- 
cussed earlier in this report. 

Pipes should be installed 6 to 12 
in from the floor and 12 to 18 in from 
the rib that has the lower elevation. 
The pipe should be at least 7 to 
in length with a 1- to 2-in ID. 



10 ft 
Both 



ends of the pipe must be threaded to 



Porous 
polyethylene tube 



Valves 



Not to scale 




Standpipe 



Pressure gauge 



FIGURE A-9. - Diagram of standpipe, pressure 
gauge, and porous tube arrangement. 



facilitate the installation of the 
following: (1) A pressure gauge and 
stand pipe, installed on the outby side 
of the bulkhead; (2) a porous tube which 
traps sediment , but allows water to pass 
through, installed on the inby side. 
Figure A-9 shows this arrangement in 
detail. 

The pressure gauge measures hydrostatic 
pressure on the bulkhead and should be 
able to register at least 50 psi. The 
standing pipe indicates the exact water- 
head behind the bulkhead in a direct 
height-to-height relationship. It can be 
constructed from any clear plastic tube, 
such as plexiglass. It should be in- 
stalled vertically to roof level with a 
valve, capable of withstanding a static 
pressure greater than 50 psi, separating 
the pressure gauge from the standing 
pipe. 

The porous tube used in tests was made 
of polyethylene and acquired from Piezom- 
eter Research and Development Corp. Fig- 
ure A-10 shows this tube installed on the 
inby side of the bulkhead. The tube al- 
lows water to pass without affecting 
pressure readings, and traps sediments 
that may clog the standing pipe and pres- 
sure gauge. 

The valve that separates the standing 
pipe from the pressure gauge should re- 
main closed until use. To determine the 
waterhead behind the bulkhead, open the 
valve and measure the height of the water 
in the standing pipe. If the water over- 
flows from the standing pipe, the water 
behind the bulkhead has reached at least 
roof level. If this is the case, close 
the valve so that the waterhead can be 
determined from the pressure gauge. Read 
the gauge, then divide the reading by 
0.434 to determine the waterhead behind 
the bulkhead. 



35 





-o 

D 



=1 



CD 

-o 






o 

Q. 

(D 

c 

CD 



CD 
_>. 

O 

CL 



LU 
O 



36 



APPENDIX B.— FLEXURAL STRENGTH ANALYSIS FOR CONCRETE BLOCK BULKHEAD 



Properly constructed bulkheads that 
are fixed on at least three sides (both 
ribs and floor) when subject to a hydro- 
static pressure are most likely to fail 
in flexure (22). 1 The flexural stress on 
the bulkhead is given by the following 
equation (23) : 

F + =J^i CA-1) 



where F + = the flexural stress on the 
bulkhead, psi; 

6 = correction factor, no units; 

p = hydrostatic pressure, psi; 

b = bulkhead height, in; 

and T = bulkhead thickness , in. 

P is a correction factor dependent upon 
the width-to-height ratio of the bulkhead 
and the particular loading condition. 
Figure B-l (22) gives the correction fac- 
tor for various waterheads, H. Using the 
maximum condition given H = b, and a 
width-to-height ratio of a/b =3, p is 
approximately 0.80. Substituting into 
equation A-l and solving for F-)-: 

F + =&^ 
*+ T 2 

where p = 0.80; 

p = 2.6 psi (6 ft of waterhead); 

Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendixes. 



and 



b = 72 in; 
T = 16 in. 



therefore F + = 42.1 psi. 

The maximum allowable flexural stress 
for nonreinforced masonry unit con- 
structed of solid block is 40 psi as rec- 
ommended by the ACI Code (24). There- 
fore, the maximum allowable pressure that 
the bulkhead described in appendix A can 
safely withstand, keeping within the lim- 
its of the ACI Code, is approximately 2.6 
psi (6 ft of waterhead). This maximum 
allowable pressure may seem unreason- 
ably low when compared with the ultimate 
pressure of 40 psi that the bulkhead 
resisted. But, it must be realized that 
the time duration of tests was very 
short when compared with actual mine 
conditions. This analysis neglects the 
pilaster center, the gunite coating, and 
the transverse pattern of laying the 
block. These design features are diffi- 
cult to access but would provide addi- 
tional resistance to flexural failure. 




FIGURE B-l. - Correction factor for bulkhead width- 
to-height ratios. Adopted from F. S. Kendorski, I. Khos- 
la, and M. M. Singh (22). 



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